The present invention is concerned with an improved pneumatic positioner which utilizes a cantilever beam spring as the balancing force in the feedback mechanism of a pneumatic positioner, and which is arranged to compensate for and eliminate lateral forces on the spool valve conventionally employed in the positioner mechanism.
Pneumatic positioners are, in themselves, well known, and constitute mechanism devices which provide the required control to position valves in a throttling mode. More specifically, pneumatic positioners operate to position a valve in accordance with a predetermined mathematical relationship to an analog pneumatic pressure signal. The force created by the analog signal acts on a diaphragm, which is, in turn, coupled to a spool valve in the positioner mechanism for displacing the spool valve thereby to effect rotation of a valve actuator and a valve connected thereto, and the signal pressure acting on the diaphragm is balanced against another force which is controlled by a feedback loop reacting to the position of the valve being controlled. When these two forces are in balance, a null point is reached and the positioner stops the motion of the valve. This position control can be accomplished by a spool valve opening or closing the flow of air to a pneumatic actuator mounted directly on the valve.
Arrangements of the general type discussed above normally employ a coil spring in the feedback mechanism of the positioner. Typical such positioners employing coil springs are shown in the U.S. Pat. Nos. to Barlow 3,954,045, Wittren 3,511,134, Baumann 3,293,992, and Ray 3,151,531. These references illustrate other features of such positioners e.g., the possibility of employing an electrically controlled solenoid as the transducer rather than a pneumatic transducer responsive to signal air pressure.
Known systems of the type described above fail to provide a number of highly desirable features. By way of example, and without any attempt to be all-inclusive, it is desirable that the positioner have as few parts as possible and, more particularly, as few pivot points as possible, since any wear in the feedback mechanism of the positioner will result in inaccuracies of the performance of the positioner. The fewer the pivot points, the smaller the wear and therefore the smaller the backlash. It is desirable, moreover, that the mechanism be capable of repeating a desired control operation without frequent calibration or readjustment. In addition, the positioner should have the lowest cost possible without sacrificing performance, ruggedness or reliability.
These highly desirable features are all achieved in the present invention and, more particularly, are achieved by elimination of the coil spring feedback mechanism which has typically been employed heretofore, and the replacement of such a coil spring mechanism by a cantilever spring beam arrangement. Spring beams, per se, have been employed heretofore in other types of control mechanisms, e.g., mechanisms of the type shown in Jenney, U.S. Pat. No. 3,455,318 and Gordon et al, U.S. Pat. No. 3,339,572. However, the arrangement employed in the present invention is quite different from those employed in these prior devices.